|
|
ISDN User Part
The ISDN User Part (ISUP) defines the protocol
and procedures used to set-up, manage, and release trunk circuits
that carry voice and data calls over the public switched telephone
network (PSTN). ISUP is used for both ISDN and non-ISDN calls.
Calls that originate and terminate at the same switch do not
use ISUP signaling.
Basic ISUP Call Control
Figure 8 depicts the ISUP signaling associated
with a basic call.
- When a call is placed to an out-of-switch number, the
originating SSP transmits an ISUP initial address message
(IAM) to reserve an idle trunk circuit from the originating
switch to the destination switch (1a). The IAM includes
the originating point code, destination
point code, circuit identification code (circuit
"5" in Fig. 8), dialed digits and, optionally,
the calling party number and name. In the example below,
the IAM is routed via the home STP of the originating switch
to the destination switch (1b). Note that the same
signaling link(s) are used for the duration of the call
unless a link failure condition forces a switch to use an
alternate signaling link.
Figure 8. Basic ISUP Signaling
- The destination switch examines the dialed number, determines that it serves
the called party, and that the line is available for ringing. The destination
switch rings the called party line and transmits an ISUP address complete message
(ACM) to the originating switch (2a) (via its home STP) to indicate that the
remote end of the trunk circuit has been reserved. The STP routes the ACM to
the originating switch (2b), then the terminating switch provides power ringing to the called party and audible ringing tone to the calling party.
In the example shown above, the originating and destination switches are directly
connected with trunks. If the originating and destination switches are not directly
connected with trunks, the originating switch transmits an IAM to reserve a
trunk circuit to an intermediate switch. The intermediate switch sends an ACM
to acknowledge the circuit reservation request and then transmits an IAM to
reserve a trunk circuit to another switch. This processes continues until all
trunks required to complete the voice circuit from the originating switch to
the destination switch are reserved.
- When the called party picks up the phone, the destination switch terminates
the ringing tone and transmits an ISUP answer message (ANM) to the originating
switch via its home STP (3a). The STP routes the ANM to the originating
switch (3b) which verifies that the calling party's line is connected
to the reserved trunk and, if so, initiates billing.
- If the calling party hangs-up first, the originating switch sends an ISUP
release message (REL) to release the trunk circuit between the switches
(4a). The STP routes the REL to the destination switch (4b). If
the called party hangs up first, or if the line is busy, the destination switch
sends an REL to the originating switch indicating the release cause (e.g., normal
release or busy).
- Upon receiving the REL, the destination switch disconnects the trunk from
the called party's line, sets the trunk state to idle, and transmits an ISUP
release complete message (RLC) to the originating switch (5a)
to acknowledge the release of the remote end of the trunk circuit. When the
originating switch receives (or generates) the RLC (5b), it terminates
the billing cycle and sets the trunk state to idle in preparation for the next
call.
ISUP messages may also be transmitted during the connection phase of the call
(i.e., between the ISUP Answer (ANM) and Release (REL) messages.
ISUP Message Format
ISUP information is carried in the Signaling
Information Field (SIF) of an MSU. The SIF contains the
routing label followed by a 14-bit (ANSI) or 12-bit (ITU)
circuit identification code (CIC). The CIC indicates
the trunk circuit reserved by the originating switch to carry
the call. The CIC is followed by the message type field
(e.g., IAM, ACM, ANM, REL, RLC) which defines the contents of
the remainder of the message (Fig. 9).
Figure 9. ISUP Message Format
Each ISUP message contains a mandatory fixed part containing mandatory
fixed-length parameters. Sometimes the mandatory fixed part is comprised only
of the message type field. The mandatory fixed part may be followed by the mandatory
variable part and/or the optional part. The mandatory variable part
contains mandatory variable-length parameters. The optional part contains optional
parameters which are identified by a one-octet parameter code followed by a length
indicator ("octets to follow") field. Optional parameters may occur
in any order. If optional parameters are included, the end of the optional parameters
is indicated by an octet containing all zeros.
Initial Address Message
An Initial Address Message (IAM) is sent in the "forward"
direction by each switch needed to complete the circuit between
the calling party and called party until the circuit connects
to the destination switch. An IAM contains the called party
number in the mandatory variable part and may contain the calling
party name and number in the optional part.
Figure 10. ANSI and ITU-T Initial Address Message (IAM)
Format
Continue with
ISUP Part 2...
|
